OK, we all know this happens and that we should always allow for this to happen in our projects.

BUT how much:

per inch of width?

per Species of wood?

in the deep South?... of far North?

All of the literature says to allow for this, but I am lost as to finding any reference source that tells me that a 9in(or 4in, etc) wide pine board will need xxx fraction of an inch of space for expansion/contraction.

Help me out folks, is there a source I am missing, you know… that one source that eludes me?